diff --git a/lib/snowpacker/env.rb b/lib/snowpacker/env.rb
index c9bfabd..daeffad 100644
--- a/lib/snowpacker/env.rb
+++ b/lib/snowpacker/env.rb
@@ -13,7 +13,7 @@ def set_env_variables
       private
 
       def set_env(env_var, value)
-        ENV["#{ENV_PREFIX}_#{env_var}"] = value.to_s
+        ENV["#{ENV_PREFIX}_#{env_var}"] ||= value.to_s
       end
     end
   end